1. The Field of the Invention
The present invention relates generally to non-volatile, static random access memory (nvSRAM) and, more specifically, to providing an erase voltage to the non-volatile SONOS transistors of an nvSRAM so as to prevent under/over erasure.
2. The Relevant Technology
In general, a computer system is comprised of a memory for holding data and programs, a processor for executing the programs or operating on the data held in memory, and an input/output device for facilitating communications between the computer system and a user. There are several different types of digital memories available for use in the memory portion of a computer system. In many instances, the particular application in which the computer system is intended to be used dictates the type of memory that is appropriate for all or a portion of the memory of the computer system. For instance, one application for a computer system in which an nvSRAM may be appropriate is in a portable computer system. Portable computer systems are generally designed to operate, if needed, with power supplied by a battery housed within the system. If the battery becomes incapable of providing power to the system and an alternative source of power is not available, the data held in memory could be come irretrievably lost. In such applications, it is desirable to use an nvSRAM because the static random access memory (SRAM) portion is capable of retaining the data while power is present and providing the performance needed during normal operations, and the non-volatile memory (nv) portion is capable of retaining data for an extended period of time after power has been removed and once power is restored, reestablishing the data in the SRAM portion.
A basic nvSRAM memory device is comprised of (1) a plurality of nvSRAM memory cells; and (2) a controller for managing the operations of the nvSRAM memory cells. The nvSRAM cell is comprised of a static random access memory cell and a non-volatile memory cell. Briefly, the terms “random access memory” and “RAM” refer to the ability to access any one of a plurality of cells in the memory at any time to write/read data to/from the accessed cell. In contrast, other types of memory require that other memory locations be traversed before the desired memory location can be accessed. These types of memories (magnetic tape, for example) are typically much slower than a random access memories. The term “static” refers to the ability of the memory to retain data as long as power is being supplied. In contrast, the term “dynamic” refers to memories that retain data as long as power is being supplied and the memory is periodically refreshed. The term non-volatile refers to the ability of a memory cell to retain data in the absence of power.
An nvSRAM typically utilizes silicon/oxide/nitride/oxide/silicon (SONOS) nonvolatile cells to provide data retention to a standard high speed SRAM memory when the power is removed. An nvSRAM provides a solution in electronic systems where it is desirable to retain the configuration of the system just before a power failure event, such as redundant array storage. Other applications include high speed data logging with nonvolatile storage in cases where Flash or EEPROM memories are limited by the number of writes required, or by the slow write speed of these technologies. One significant advantage of an nvSRAM, compared with other nonvolatile technologies, is unlimited reads and writes to the SRAM portion of the nvSRAM since the data is written to non-volatile storage elements only upon power down. The nvSRAM is accessed with standard SRAM interfaces and timings; the non-volatile operations are transparent to the user. The non-volatile operations are typically accomplished by transferring data from active SRAM latches into a separate set of SONOS non-volatile cells connected by the appropriate circuitry.
The programming and erase voltages for the SONOS transistors of an nvSRAM vary in both duration and magnitude. For example, the typical erase voltages can range from −11.5 to −8.9V, 9.6 mS for SONOS transistors. The range of erase voltages on a SONOS transistor is due at least in part to a number of SONOS transistor and charge pump process differences and design marginalities that occur from lot-to-lot, wafer-to-wafer and die-to-die. A single erase voltage for all die would leave some transistors under-erased and some transistors over-erased.
Although nvSRAMs possess many desirable properties, there are limitations to its non-volatile properties. Principal among these limitations are the number of nv stores (endurance) and data retention lifetime. A typical non-volatile memory device will specify both endurance and data retention and guarantee these in its data sheet through extensive testing. Both endurance and data retention are affected by fabrication processes and process control as well as program and erase conditions of the non-volatile cells.
There are also two additional failure modes associated with nvSRAMs: failure to store at extreme cold temperatures and pre-latching failures during NV recall. Both types of failures can be addressed by choice of erase and programming voltages, as can endurance and data retention.
It is therefore desirable to provide a method for measuring the under-erase and over-erase condition of a plurality of SONOS transistors on a die and thereafter implement a proper erase voltage so as to prevent under/over erasure. It is also desirable to define program and erase conditions for an nvSRAM such that process variation will be negated while ensuring specified endurance and data retention.